Alternaria cucumerina (Ellis & Everh.) J.A. Elliott, (1917) var. cucumerina

Alternaria leaf blight

- classification : Fungi, Ascomycota, Dothideomycetes, Pleosporomycetidae, Pleosporales, Pleosporaceae
- teleomorph: Lewia
- synonymes : Alternaria cucumerina (Ellis & Everh.) J.A. Elliott, (1917) ;  Alternaria brassicae var. nigrescens Peglion, (1892) ;  Macrosporium cucumerinum Ellis & Everh., (1895)
- English names: Alternaria leaf blight, Alternaria wilt


Alternaria is a leaf disease that affects many species of Cucurbitaceae belonging to the genera Citrullus, Cucumis, Cucurbita . The responsible fungus, Alternaria cucumerina , appears to be widespread around the world. It is reported for example on the American continent (Canada, USA, Venezuela, Brazil, Argentina, Chile), Cuba and Trinidad, Australia, New Zealand, Asia (Japan, China, Thailand, India, Arabia ), in Africa (Egypt, Kenya, Libya, Mozambique, Nigeria, Rhodesia, South Africa, Sudan, Zambia), in Cyprus. In Europe, it would have been observed in particular in England, Romania and France . Note that to our knowledge, this fungus does not seem to have raged seriously in our territory for many years.

As we suggested earlier, its host range in Cucurbitaceae is quite wide, affecting cucumber, melon , watermelon, Cucurbita moschata , Luffa aegyptiaca and Luffa cylindrica , ( Momordica charantia bitter gourd ), cristophine .


Main symptoms

A. cucumerina is primarily a pathogenic fungus on leaves , although symptoms in fruits are also mentioned. Rather, the first lesions appear on old leaves near the crown. They are initially small (0.5 mm in diameter), brown-yellow in color, and bordered by a light green to yellow halo. Subsequently, these spots gradually spread, coalesce, and form large necrotic and brown areas that can reach several centimeters. Note that these lesions show more or less concentric patterns by transparency. Eventually, many leaves necrotize and die, leaving the fruits exposed to sunburn . Let us add that the stems or the petioles do not seem to be affected.

Concave and concentric lesions, several centimeters in diameter, can also be observed on fruits ripe . The injured tissues are often covered with a dark olive green felting consisting of the conidiophores and conidia of A. cucumerina . This fungus can also cause such damage during transport and storage of fruit, especially on melons and squash.


Biology, epidemiology

• Conservation, sources d'inoculum

A. cucumerina persists in a plot thanks to its mycelium (and probably chlamydospores formed from it) which can survive in the debris of diseased plants for at least two years. Its conidia, although durably surviving drought for several months, seem to quickly lose their viability in the soil. The seeds are also likely to preserve, and therefore to disseminate this fungus, note that the contamination does not appear to be internal. A. cucumerina would also be lodged by some cultivated or wild plants which could ensure its multiplication and its conservation.

• Penetration, invasion

The dormant mycelium present in the plant debris appears to constitute the primary inoculum. If conidia polluting the surface of the lamina perform this role, they emit one to several germ tubes. Subsequently, the fungus directly penetrates the cuticle, or either through the stomata or from a wound. In hot and humid conditions, infection would take place a maximum of 46 hours + after germination of conidia. The mycelium once in place in the tissues invades them and gradually destroys them. Incubation lasts between 3 and 12 days depending on climatic conditions.

• Sporulation and dissemination

A. cucumerinum sporulates more or less abundantly on altered tissues in humid conditions and forms conidia which will ensure the dissemination of the disease, and therefore secondary contaminations. Indeed, these spores, once dry, are easily transported by wind and air currents over significant distances and / or by splashing water.
It should be remembered that the seeds can also be the source of the spread of early blight.

• Conditions favorable to its development

This fungus is particularly prevalent in rainy production areas where temperatures are high. The presence of free water on the leaves (following dews, rains, sprinkling) and temperatures between 21 and 32 ° C are very favorable to contamination and the formation of conidia. The germination of the latter requires the presence of free water on the leaves and it can occur between 10 and 35 ° C, with an optimum located around 25-28 ° C. The severity of the disease increases in proportion to the duration of wetness of the leaves. It would be favorably influenced by frequent rains which would increase the relative humidity of the canopy, and would accentuate the dispersion of conidia by splashing. Finally, it should be noted that the proliferation of leaf miners is the cause of an increase in the number of early blight lesions on melon leaves.


Protection methods

• During cultivation

Early blight is a disease that does not always require the application of fungicides. Indeed, very often its incidence is rather low and does not justify treatment. However, if this disease is growing in crops, several fungicides are reported as effective in the literature: chlorothalonil, maneb mancozeb, iprodione imazalil, azoxystrobin. It should be noted that this disease does not seem to be rampant in France at present and that no fungicide is currently approved on Cucurbitaceae.

On the other hand, a certain number of measures must be implemented in order to limit its extension. From the first symptoms, it is advisable to avoid the presence of free water on the plants and high humidity. Under shelter, it is necessary to ventilate as much as possible and to ban watering and irrigation by sprinkling.
In the open field, be careful not to use sprinkler irrigation. If it is essential, it will be carried out in the morning or during the morning so that the foliage wears off quickly, in no case in the evening.
It will be essential to avoid any stress to the plants, and to ensure them a balanced manure, in particular in nitrogen.
As much as possible, as much plant debris as possible (leaves and fruits) should be removed during and at the end of the crop, and destroyed. Some authors advise burying them deeply.

• Next crop

The rotations of at least 2 years with non-host plants will reduce inoculum levels in the soil. Weeds that can serve as intermediate hosts should also be eliminated.
In addition, avoid setting up a crop near plots of cucurbits already affected, or other sensitive crops such as potato or eggplant.
The manures will be balanced, and it will be advisable to avoid the excess or the lack of nitrogen.
It will also be wise to plant at a density that allows good aeration of the vegetation, good wiping after rains or sprinkler irrigation, and to produce rather vigorous plants. Avoid planting in hydromorphic soils. In the presence of such soils, it will be advisable to drain them. It will be better to irrigate the plants by drip rather than by sprinkling.

Some fungicides can only be achieved by serious epidemic situations, with the active substances previously reported.
The application of sodium, potassium and ammonium bicarbonates on melon would have made it possible to control more or less effectively the development of this Alternaria on this cucurbitaceae. In addition, it will be necessary to limit the development of leafminer epizootics which seem to favor the expression of early blight.

The resistance varietal more or less effective were detected in some types of melons (cantaloupe, honeydews), they do not seem to currently available in commercialized varieties.

* Chemical control : As the number of pesticides available for a given use is constantly changing, we advise you to always confirm your choice by consulting the e-phy site of the Ministry of Agriculture and Fisheries which is an online catalog of plant protection products and their uses, fertilizers and growing media approved in France. This also applies to all biological products based on microorganisms or natural substances.